INFLUENCE OF SO2 ON THE NO/N2O CHEMISTRY IN FLUIDIZED-BED COMBUSTION .2. INTERPRETATION OF FULL-SCALE OBSERVATIONS BASED ON LABORATORY EXPERIMENTS

In circulating fluidized bed combustion of coal, significant interactions between desulfurization by limestone and emissions of nitrogen oxides (NO, N2O) are generally observed. In order to facilitate interpretation of experimental results obtained in a 12 MW circulating fluidized bed boiler, a series of laboratory tests have been carried out. The tests include homogeneous CO and HCN oxidation in the presence of different concentrations of SO2, CO oxidation catalysed by calcined and partly sulfated limestone particles, and formation and reduction of NO and N2O over bed material containing char. The presence of SO2 in the gas is shown to decrease the rate of homogeneous CO and HCN oxidation and thereby change the product distribution of the nitrogen-containing species. Unlike sulfated limestone, calcined limestone (CaO) is shown to be a good catalyst for oxidation of CO. A lower mean CO concentration is therefore expected in fluidized bed combustors during injection of limestone. This indirectly influences the nitrogen chemistry. Finally, the influence of NO and O2 on the formation of N2O from char was studied.